Generally, in a semiconductor process, alignment of wafers to each other is critical during the bonding step. It is noted that the common alignment system of an exposure apparatus used in the manufacture of semiconductor devices is capable of detecting alignment marks or reticle provided on a mask and a wafer so as to bring the mask and the wafer into a predetermined positional relation on the basis of the detected positional relationship between the mark and wafer, wherein the mask is usually transparent. Nevertheless, in additional to a simple stacking and bonding wafers in precision alignment, wafer bonding in the Micro-Electro-Mechanical Systems (MEMS) field may further consist of the bonding of face-to-face and face-to-back wafer stack. Currently, most bonding devices adopted the concept of the conventional exposure apparatus for aligning wafers to be bonded, that is, a first wafer with alignment marks etched thereon is first fed into a bonding device for detecting and recording the image of the first wafer by a photo-detecting means, and then a second wafer with alignment marks etched thereon is fed into the bonding device to be detected by the photo-detecting means and thus for bonding with the first wafer by aligning the second wafer with the recorded image of the first wafer. Moreover, a variation of the foregoing alignment process is achieved by simultaneously detecting the two wafers to be bonded by a combination of lens of the photo-detecting means. Also for enabling the simultaneously detection and alignment of wafers to be bonded, an infrared light is used for “see through” the wafers and acquiring images thereof since most devices are adapted for wafers having alignment marks etched on a surface thereof. The alignment error of these systems can be huge.
Please refer to FIG. 1, which is a schematic view of an alignment system as disclosed in U.S. Pat. No. 5,166,754. The alignment of an upper wafer 10 and a lower wafer 12 is performed by the sequence of: using a coupled charged device (CCD) lens 14 to detect an alignment mark 100 disposed on the upper wafer 10 through a via hole disposed on the lower wafer 12; and then bringing the upper wafer 10 and the lower wafer 12 into a predetermined positional relation on the basis of the detecting of the alignment marks 100 through the via hole 120 by using a adjustment device (not shown).
In another prior-art alignment system as shown in FIG. 2, an alignment mark 200 is disposed on an upper wafer 20 while an alignment marks 220 is disposed on a lower wafer 22 such that the alignment of the upper wafer 20 and the lower wafer 22 is performed by the sequence of: feeding the upper wafer 20 to a predefined position for enabling a CCD to detect and record the location of the alignment mark 200; feeding the lower wafer 22 to the predefined position for enabling the CCD to detect and record the location of the alignment mark 220; and then, bringing the upper wafer 20 and the lower wafer 22 into a predetermined positional relation on the basis of the detected positional relationship between the alignment marks 200, 220 by using an adjustment device (not shown).
It is yet another prior-art alignment system as shown in FIG. 3, where an CCD 34 is arranged at a predetermined position enabling the same to be able to detect an alignment mark 300 of an upper wafer 30 and an alignment mark 320 of a lower wafer 32 simultaneously so as to bring the upper wafer 30 and the lower wafer 32 into a predetermined positional relation on the basis of the detected positional relationship between the alignment marks 300, 320 by using an adjustment device (not shown).
Further, another prior-art alignment system disclosed both in U.S. Pat. Nos. 6,479,371 and 4,019,109 is shown in FIG. 4, in which both the alignment mark 400 disposed on a upper wafer 40 and the alignment mark 420 disposed on a lower wafer 42 are irradiated by a penetrating light source, such as X-ray, for enabling a CCD 44 to be able to detect the alignment mark 400 and the alignment mark 420 simultaneously so as to bring the upper wafer 40 and the lower wafer 42 into a predetermined positional relation on the basis of the detected positional relationship between the alignment marks 400, 420 by using an adjustment device (not shown).
Please refer to FIG. 5A to FIG. 5C, which are schematic diagrams showing an alignment sequence performed by an alignment system with dual-CCDs according to prior arts. The alignment of an upper wafer 50 and a lower wafer 52 is performed by the sequence of: using an upper coupled charged device (CCD) 54 to detect and record an alignment mark 520 disposed on the lower wafer 52 while the upper wafer 50 is at a withdrawn position as shown in FIG. 5A; feeding the upper wafer 50 to a predefined position while withdrawing the lower wafer 52 for enabling a lower CCD 56 to detect and record an alignment mark 500 disposed on the upper wafer 50 as shown in FIG. 5B; bringing the upper wafer 50 and the lower wafer 52 into a predetermined positional relation on the basis of the detected positional relationship between the recorded alignment marks 500, 520 by using an adjustment device (not shown). Accordingly, the actuating error introduced by the mechanism that brings the two wafers in contact can be minimized since the initial interval between the two wafers 50, 52 is minimized.
From the above description, it is noted that the actuating error still can not be avoid while the two wafer is brought into contact by a mechanism. Therefore, a method for stacking and bonding wafers in precision alignment is required.